Recovery of oil from oil fields



Sept 18, 1945. M, M SKAT 2,385,298

RECOVERY OF OIL FROM OIL FIELDS Filed Oct. 16, 1941 J 011. AND GAS GASZONEll GAS-IEL CONTHGT' FLA J 3 YVU nwvbon Morrjs Mus/Quiz STRAINER-JSPatented Sept. 18, 1945 RECOVERY OF OIL FROM OIL FIELDS Morris Muskat,Oakmont, Pa., asslgnor to Gull Research & Development Company,Pittsburgh, Pa., a corporation of Delaware Application October 16, 1941,Serial No. 415,295

4 Claims.

. This invention relates to improving recovery of oil from oil fields.

In producing oil fields, the force that drives oil from the permeablereservoir rock containing it to the wells is pressure exerted on the oilin the rock by some fluid other than oil. Without such extraneous drivefluid the oil, which saturates the rock in the manner of a sponge,remains stagnant. The drive fluid may be compressed natural gasoccurring naturally in association with the oil, or gas or airartificially injected under pressure. Or it may be water underhydrostatic pressure, the water being either naturally present orartificially introduced. To produce oil it is essential that suchextraneous fluid pressure be maintained. The amount of oil that can berecovered from a, producing formation depends directly upon the degreeto which the gas associated with the oil within the oil bearing strataor contiguous strata is conserved. That is, the lower the average volumeof gas which is produced with each barrel of oil the higher will be thecumulative recovery of oil from the producing formation.

in the best modern oil field development technique gas pressure ismaintained as long as possible by various expedients. Sometimes gasproduced with oil is separated and fed back into adjacent wells underpressure. And, when an oil field has become "exhausted? by reason ofloss of gas, oil can still be produced by injecting gas, or air, insuitable ways, to replace the lost forma- 'tion pressure.

The great difficulty in maintaining the required gas driving pressure onthe oil is that the gas which flows much more readily through rock thanoil does anyway, tends to short circuit the 011 beds and escape. Thatis, gas under pressure instead of performing its duty of-squeezing oilout of an adjacent oil sand takes the path of least resistance and leaksaway through the most permeable bed it can find.

Probably the greatest problem in operating an oil field is that ofkeeping down the ratio of extranecus fiuid, to oil, in the fluid mixtureproduced from the wells.

Ordinarily the gas-bearing Stratum overlies the oil strata, or at leastoverlies the main ones. This is especially true in oil deposits of thegascap type, where the reservoir may be visualized as animpervious-roofed dome, partly filled with oil and overlain by a body ofcompressed gas at pressure equal to the hydrostatic pressure of the.

oil. As the oil production proceeds, the body of gas usually expands andtravel outwards and downwards toward the .iianks of the structure,

- increasing the original thickness of free gas horizons at any onplace, and creating new gas cap extensions into parts of the stratawhich originally may have been fully saturated with oil. A similarsituation obtains even in fields which do not originally possess gas capzones, but in which during the course of production 0! oil and gas theupper parts of the pay become greatly depleted of the oil and entrap theupward migrating gas phase, thus creating free gas zones which areessentially equivalent to those occurring in normal gas caps.

In conventional oil production practice, the only expedient of muchpractical use for keeping down gas influx has been to carry the tubingto the very bottom of the well, so that gas from the overlying gasformation has to take as long a passage as possible before reaching thetubing intake. This is not very eilective.

Other methods have been devised with the accomplishment of that end inview. involving mechanically isolating the productive strata whileshutting oil the overlying gas strata. These known methods aresatisfactory provided the levels of the several strata, and certainother important conditions, are accurately known. But often suchknowledge is impossible of attainment.

The main object of the invention is the provision of a means forlowering the production ratio of gas to oil in recovering oil fromsubterranean deposits; without requiring an accurate knowledge ofsubsurface stratigraphy or indeed of several other conditions,information as to which has hitherto been essential.

The invention is based on the discovery that a special kind of choke canbe provided in wells in a manner such that in eilfect it throttles downthe gas how to a marked degree and at the same time has but littlethrottling effect on the oil flow. In the simplest embodiment the choketakes the form of an annular body of longitudinally and laterallypermeable character, so disposed with respect to the well tubing and tothe productive and gas-bearing strata faces that the gas has to take amuch longer obstructed path than the oil from or to a well. The resultis a sort of selective choking action which brings about a remarkablereduction in gas-oil ratio.

The invention is applicable to both input wells and output wells. Forthe sake of con creteness it will be described primarily in connectionwith output wells.

The mode of practicing the invention will be clear from a considerationor the accompanying drawing which is a schematic view in verticalsection of the invention embodied in an output well, for the purpose ofconserving natural formation pressure.

s shown, a well l penetrates a gas cap or stratum ll, and extends downto an oil-producing stratum I 2. The gas-oil contact plane is shown at13. A tubing I4, with a strainer-protected inlet ii at its bottom,extends 'to" the bottom of the oil stratum. Oil is produced through thetubing by pump, gas-lift or other means not shown.

Assuming the hydrostatic head of oil in the tubing to be less than theformation pressure in stratum l I, gas tends to escape down the annularspace between tubing and well, to the tubing inlet, and to be producedalong with the oil. In ordinary practice inordinate amounts of gas maybe produced along with the oil. It would seem easy to shut oil the gasstratum in some way, and thus solve the problem, but in practice this isdiflicult or impossible. Things are not so clearcut as appears in apictorial representation. Even by the most careful rock coring, or useof electric log ing methods, it is often impossible to locate the gasstrata with any certainty.

According to the invention an annular permeable choke I6 is provided inthe space between the tubing and the well, extending throughout theheight of the formations which produce oil or are believed to produceoil. This choke can take many forms. It can be a monolithic mass such aspermeable cement, or, more conveniently, a mass of non-consolidatedparticles such as sand.

.In use the choke imposes very little restriction on oil flow, as only ashort passage of oil through the choke is required. But the throttlingeffect on the gas flow, which may have to course through many yard ofthe choke, is considerable.

If the producing pressure differential over the oil formation be keptless than the fluid head between the top of the perforationsin thetubing and the gas-oil contact plane the escape of gas from the free gaszone will be completely eliminated. And if the producing pressuredifferential exceeds this fluid head the escape of gas from the free gaszone is severely limited and reduced in quantity. As a matter of fact,from the mechanics of the flow of petroleum fluids through porous mediait is apparent that a restricted column of porous material such as thatwhich is disposed between the tubing and the exposed face of theproducing strata in the present invention will increase the resistanceto flow from a gas-producing formation by an amount that is quitecomparable to the total resistance of the reservoir sand itself.

It will be seen that .in so positioning the choke with respect to theoil and gas-producing formations and to the inlet of the productiontubing it is not necessary to predetermine the position of the gas-oilcontact. The method automatically restricts the escape of gas from thefree gas zone regardless of the position of the gas-oil contact providedonly that the choke be extended to the plane of gas-oil contact.Preferably, it can extend to the very top of the gas bearing strata.

While the presence of the choke will have some tendency to increase theresistance to the flow of the oil from the oil bearing zone to thetubing, this efiect is quite negligible unless the permeabiilty of thechoke material be very low as compared to the permeability of the mainbody 01' the producing formation. In order to minimize even this smallincreased. resistance to the flow of the 011, my invention encompassesthe setting of the choke in such a way that gravel rather than sand isplaced opposite the oil bearing strata. and that sand be set on top ofthe gravel and opposite the gas bearing strata. This or any othercombination of sand and gravel or either separately may be used toeil'ect the pur- Dose of this invention.

Placement of the choke is usually simple.

' Using sand for the choke, the sand can be deposited with the aid ofthe apparatus shown in Chenault Patent 2,140,072. II a consolidated(monolithic) choke is desired, it is convenient to employ Portlandcement containing enough gas generating agents (advantageously hydrogengenerating agents) to give to the mass the required permeability underth'e conditions in which the mass sets. Such a cement choke is set bymethods already known for making permeable cement filter packs.

In installing the tubing and choke, the tubing, with its end initiallyclosed or provided with a valve, is set to the very bottom of the wellbore, and the choke deposited around it. If desired, the lower end ofthe tubing can be replaced by a suitable liner, screen or strainer,

In employing the invention in conjunction with ga repressuring, the samearrangement is employed as shown in the drawing, but repressuring gas isforced down the tubing and thence into the rock. The'ratio of the gasentering the oilbearing stratum to that entering the bypassing stratumis increased. A similar arrangement can be provided in the output wells,to effect still further improvement in gas-oil ratio.

In practice, the annular opening between well bore l0 and tubing I 4 isclosed off somewhere above choke is in a conventional manner (notshown), thus limiting flow of fluids in the well to that entering orleaving through tubing M. For this purpose, a packer may be set anywherebetween the well head and the top of ch'oke 96, but

I often find it more advantageous to run the tubing through a stuffingbox at the well head.

What I claim is:

1. In a well extending through a gas-containing formation into a lower,oil-nearing stratum, a fluid conduit extending into the well andcoinmunicating therewith at a point the loottom within the horizons oithe oil-bearing stratum, and an elongated tubular choke made of apermeable medium enclosing the lower end of said fluid conduit andextending upwardly therefrom over the exposed face of said oil-bearingstratum and therebeyond at least to the plane of gas-oil contact.

2. In a well extending through an upper gasbearing stratum and a loweroil-bearing stratum: tubing extending down to, and opening adjacent, thebottom of said on hearing stratum, and an elongate annular choke made ofa medium permeable in all directions, filling the space between thetubing and the exposed strata faces, whereby an obstructed fluid path isprovided between the gas stratum and the tubing opening, relativelylonger than the path between the oilbearing stratum and the tubingopening.

3. In a producing well extending through an upper gas-bearing stratumand a, lower oil-bearing stratum: tubing extending down to, and openingadjacent, the bottom of said oil bearing stratum, and an elongateannular choke made of a medium permeable in all directions, filling thespace between the tubing and the exposed strata laces, whereby anobstructed fluid path is provided between the gas stratum and the tubingopening, relatively longer than the path between the oil-bearing stratumand the tubing opening. e

4. In a gas-repressuring input well extending through an uppergas-bypassing stratum and a lower oil-bearing stratum: gas input tubingextending down to, and opening adjacent, the oil bearing stratum, and anelongate annular choke made of a medium permeable in all directions,filling the space betweenthe tubing and the exposed strata faces,whereby an obstructed fluid path is provided between the gas stratum andthe tubing opening, relatively longer than the path between theoil-bearing stratum and the tubing opening.

t MORRIS MUSKAT.

